Biosecurity New Zealand says "considerable progress" has been made in the effort to eradicate the aggressive aquatic weed, salvinia, from the Wairākei Stream in Pāpāmoa.
Salvinia is one of the world's worst aquatic weeds, it can completely blanket waterways, has the potential to destroy the habitat of native plants and animals, including native aquatic bird life, and can cause a drowning hazard.
Biosecurity New Zealand, which is part of the Ministry for Primary Industries, has been working with the Tauranga City Council, the Bay of Plenty Regional Council and local iwi Ngā Potiki to remove the weed since it was detected in the stream in early 2018.
The waterway has been monitored weekly since then, including the weekly removal of every last fragment of salvinia from the water and banks.
John Sanson, manager of recovery and pest management at Biosecurity New Zealand, said during the latest inspection of the stream, no salvinia plants were found.
"This is a very promising sign and shows the eradication effort is going well."
He said weekly inspections of the waterway will continue until late April, to ensure every remaining piece of salvinia is removed, "because even very small fragments can survive and grow".
"Once the stream has been free of plants for about three months we can go to the final phase of the eradication programme. The monitoring phase will be for a further two to three years."
Sanson said the waterway will continue to be regularly checked during this time but less frequently.
"All going well we should be able to declare salvinia eradicated from the site around 2021-22."
He said salvinia had been eradicated from other sites in New Zealand and Biosecurity New Zealand was confident it could do the same at Pāpāmoa.
Salvinia is known worldwide for its fast spread and harmful impact on lakes and waterways. It forms dense mats on the water surface and is an unwanted plant and notifiable organism under the Biosecurity Act, meaning it is illegal to grow or share plants.
The Ministry for Primary Industries said last year that it suspected someone put the salvinia plants into the Wairākei Stream, possibly from their aquarium or pond.
The infestation covered about 1.4km of the stream, which runs behind the housing area.
The site is surrounded by residential properties, sports amenities and public access walkways and parks.
One section of the stream, on Parton Rd, appeared dried up over summer but the Tauranga City Council drainage engineer Peter Mora said this condition was not related to the eradication work.
"The area is a wetland, and, with the weir open and no rain, it is at normal levels."
Salvinia molesta (salvinia)
• Is known worldwide for its fast spread and significant impacts on lakes and waterways.
• Forms dense mats on the water surface and has the potential to destroy the habitat of native plants and animals, including native aquatic birdlife.
• Attracts breeding mosquitoes, removes oxygen from the water, affects recreational activities and creates a drowning risk for people and animals.